1. Field of the Invention
The present invention relates to a developing device, particularly, to a magnetic brush type developing device for electrophotography, in which an electrostatic latent image formed on a photosensitive body is developed with a developing agent of a two-component system consisting of a toner and a carrier.
2. Description of the Related Art
It was customary to use a carrier having small particle diameters in a magnetic brush type developing device included in an electrophotographic apparatus for improving the picture image quality. The carrier of small particle diameters markedly improves the picture image quality. However, the small carrier particles are attached to a photosensitive body included in the apparatus, giving rise to serious problems as pointed out below.
First of all, the total amount of the developing agent within the developing device is decreased with decrease in the carrier amount within the developing agent. As a result, an inconvenience is generated in the system for transferring the developing agent, leading to defective images.
What should also be noted is that, if the attached carrier is transferred onto a transfer paper sheet, defective images such as white dots are formed in the solid image portion. Also, the attached carrier on the transfer paper sheet gives detrimental effects to the fixing device. Further, where the output image having the carrier attached thereto is used as an original in, for example, an automatic original feeder included in a copying machine, the carrier is attached to the automatic original feeder so as to bring about an inconvenience in the system.
Further, where the carrier attached to the photosensitive body is not transferred onto a transfer paper sheet, the attached carrier is moved to reach a cleaning blade, with the result that the service life of the blade is shortened.
The problems pointed out above can be solved by removing and recovering the carrier attached to the photosensitive body before the transfer step. In fact, a carrier recovery device comprising a magnetic member is disposed between the developing section and the transfer section in some of conventional apparatuses so as to recover the attached carrier from the surface of the photosensitive body.
The conventional carrier recovery device comprises a magnetic member disposed in the vicinity of a developing roller having a predetermined magnetic field. The magnetic field of the magnetic member differs from that of the developing roller. The carrier attached to the photosensitive body is removed by the magnetic force of the magnetic member. What should be noted is that the magnetic fields produced by the developing roller and the magnetic member affect each other in a complex fashion depending on the positions of these roller and magnetic member, giving rise to difficulties as pointed out below.
A first difficulty is that the recovery device attracts the developing agent directly from the developing roller. The difficulty is generated where the magnetic force of the recovery apparatus is greater than that produced by the main magnetic pole or transfer magnetic pole of the developing roller. If the developing agent is directly attracted to the recovery apparatus, the developing agent is accumulated in the recovery device, leading to a marked reduction in the recovery rate. In some cases, the developing agent is not recovered at all. Further, if the accumulation is increased, the developing agent is locked between the developing roller and the recovery device, with the result that the developing agent drops from the developing device into the electrophotographic apparatus so as to impair the function itself of the electrophotographic apparatus.
A second difficulty relates to the transfer capability of the developing agent on the developing roller. Specifically, the magnetic force distribution on the developing roller is disturbed by the magnetic field generated from the magnetic member so as to prevent the developing roller from performing its proper function of promptly transferring the developing agent. In other words, the developing agent is not transferred satisfactorily, with the result that the output images are rendered thin or uneven. In some cases, stirring vanes mounted within the developing device are reproduced on the picture images.
To reiterate, it is known to the art to mount a carrier recovery device within the developing device included in an electrophotographic apparatus. In this case, however, the carrier recovery function is impaired or not performed at all depending on the mounting position of the recovery device. Further, the reproduced images are rendered unsatisfactory, or the function itself of the electrophotographic apparatus is impaired.